1. Field of the Invention
The present invention is generally related to an energy pulse transmitter which provides a pulse energy level output in a coded pattern and compares a received input with the coded pattern to determine if a match occurs and, more particularly, to a photoelectric sensor that provides an output of a plurality of coded pulses and receives a reflected light pattern prior to comparing the received reflected light pattern with the transmitted coded pattern.
2. Description of the Prior Art
Many different types of photoelectric detectors are known to those skilled in the art. Photoelectric detectors can be applied in several different ways. One application is referred to by those skilled in the art as a retroreflective configuration and uses a stationary reflector to reflect a light pattern back toward its source. If an object passes through a predefined detection zone, the light beam is interrupted and the presence of the object is determined by the lack of a return signal. Another type of application is called a diffuse system in which no reflector is used. Instead, the light is reflected from a reflective object to be detected and the reflected light received by the photodetector indicates the presence of the object within the predetermined detection zone. Another type of photodetector system employs a light transmitter and a remote light receiver. This is referred to by those skilled in the art as a thru-scan system. If an object breaks the light beam, it is not received by the receiver. This lack of a received signal indicates that an object is within the detection zone which is between the transmitter and the receiver.
In typical photodetector systems, a pulsed light beam is transmitted at a preselected and constant frequency. After each light pulse is transmitted, a receiver is armed for a preselected period of time, referred to by those skilled in the art as a time window. If a return pulse is received within the window, it is considered to be a valid pulse.
Photodetectors known to those skilled in the art exhibit certain problems in particular applications. Background light from external sources can cause false triggering by a photodetector. In addition, if two photodetectors are placed in close proximity to each other, the transmitted light from one detector can be inadvertently received by the other detector and false triggering will occur as a result of this arrangement.
U.S. Pat. No. 5,151,591, which issued to Johnson et al on Sep. 29, 1992, discloses an asynchronous signal interrogation circuit for a detection apparatus. The photodetector circuit is provided to interrogate incoming signals and determine whether the frequency of those incoming signal pulses is acceptable. The circuitry of the device permits a series of incoming pulses to be interrogated in order to determine whether the frequency of those pulses is acceptable and can be assumed with confidence to be emanating from an appropriate light source. Upon the receipt of a first input signal pulse, a time window is created by the circuit in order to define a period of time during which a subsequent input signal pulse is to be expected. Other than during the duration of the time window, the circuit will not accept an input signal pulse and will not count that pulse as having been received. Each properly received pulse creates a subsequent time window until a predetermine number of consecutive pulses is received during their time windows. When that predetermined number is received, a signal is provided. In a particularly preferred embodiment of this detector, the signal that the predetermined number of consecutive pulses have been received is used to lower the threshold voltage of a comparator for the purpose of facilitating receipt of subsequent signals once the appropriateness of the incoming stream of pulses is determined.
U.S. Pat. No. 5,331,150, which issued to Marsh on Jul. 19, 1994, discloses a photoelectric sensor with variable light pulse frequency. The photo sensor is provided with a control circuit that is capable of varying the frequency of light pulses which energize a light source of the sensor. When four consecutive light pulses result in an identical status of a light sensitive component, the frequency of light pulses is slowed in order to conserve energy. However, when a change in state is recognized by the circuitry of the detector, the frequency of light pulses is increased in order to more rapidly determine whether or not a change of status has actually occurred. This permits the photoelectric sensor to conserve energy while also being able to rapidly determine whether or not the light path of the photoelectric sensor has changed state from obstructed to unobstructed or, conversely, from unobstructed to obstructed.
U.S. Pat. No. 5,496,996, which issued to Barnes et al on Mar. 5, 1996, describes a photoelectric device with the capability to change threshold levels in response to changing light intensities. The photoelectric sensor is provided with the capability of calculating a threshold magnitude based on a maximum light intensity received by a light sensitive device. The maximum value of the light intensity is determined during a preselected period of time when no object is blocking the path of the light beam. A second threshold magnitude is used to assist the detector in determining the leading edge and trailing edge of a transparent object. The values of the first and second threshold magnitudes are updated dynamically as bottles pass through the path of the light beam. This permits the photoelectric sensor to react to changes in light intensity without providing false signals of bottles within the light path or missing bottles that are within the light path.
As described immediately above, several different methods have been applied in attempts to refine the methods used to detect an object within the detection zone of a photodetector. These methods typically are directed in an effort to improve the accuracy and reliability of detection for the photodetector. Some of these concepts are directed to overcoming difficulties that can be caused by the presence of background light from external sources and light interference from other photodetectors. It would therefore be significantly beneficial if a photodetector could be developed that is virtually immune to the interference from background light and other photodetectors.